1. Field of the Invention
The present invention relates to a flow rate sensor which is usually employed to measure an intake air flow rate in an internal combustion engine, particularly relates to a flow rate sensor which is used to measure the flow rate of a fluid on the basis of a heat transfer phenomenon where a heat is transferred either from a heating element or from a portion heated by the heating element to the fluid.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 8-313318 has disclosed a thermo-sensitive type flow rate sensor which is used to measure the flow rate of a fluid flowing through a predetermined flowing passage, on the basis of a heat transfer phenomenon where a heat is transferred either from a heating element or from a portion heated by the heating element to the fluid.
FIG. 36 is a front view illustrating a conventional thermo-sensitive type flow rate sensor disclosed in Japanese Unexamined Patent Publication No. 8-313318. FIG. 37 is a cross sectional view of the thermo-sensitive type flow rate sensor of FIG. 36.
Referring to FIGS. 36 and 37, a detecting pipe conduit 19, which is formed with a bell mouth portion, is located within a main fluid passage 16 through which a fluid (whose flow rate is to be measured) flows. The fluid flows from the left to the right in FIG. 37 through the main fluid passage 16, and a flow rate detecting element 12 is disposed within the detecting pipe conduit 19.
The flow rate detecting element 12 is comprised of a ceramic substrate and a platinum layer formed by depositing platinum served as a thermo-sensitive electrically resistant material on the surface of the ceramic substrate. The thermo-sensitive electrically resistant material has a property whereby the electric resistance will change with changes in temperature. Further, the platinum layer is formed into a tooth pattern (a meander pattern) so as to serve as a flow rate detecting resistance 11. Moreover, a fluid temperature compensating resistance 13, which is used to compensate a temperature change of the flowing fluid, is also made of a platinum which is the thermo-sensitive electrically resistant material, and is disposed upstream of the detecting pipe conduit 19. A fluid rectifying grating means 17 is made of a resin and is formed into a honeycomb structure. Such fluid rectifying grating means 17 is positioned close to the inlet of the main fluid passage 16.
An electronic circuit case 15 accommodating an electronic circuit board 14 is provided on the outside of the main fluid passage 16. Mounted and fixed on the electric circuit board 14 is an electronic circuit for calculating the flow rate of a flowing fluid. In practice, the electronic circuit is electrically connected with both the flow rate detecting resistance 11 and the fluid temperature compensating resistance 13.
Referring now to FIG. 36, there is provided a connector 18 which is used to supply an electric power from the outside of the main fluid passage 16 to the flow rate sensor, and to obtain a flow rate signal from the flow rate sensor so as to send the flow rate signal to a predetermined place outside the main fluid passage 16.
In use of such conventional thermo-sensitive type flow rate sensor 1, an electric current flowing into the flow rate detecting resistance 11 of the flow rate detecting element 12, is controlled by the electronic circuit attached on the circuit board 14, in a manner such that an average temperature of the flow rate detecting resistance 11 will rise to a predetermined value which is 200.degree. C. higher than a fluid temperature detected by the fluid temperature compensating resistance 13. In more detail, when a flowing fluid quantity is small, an amount of heat being transferred from the flow rate detecting resistance 11 to the flowing fluid will also be small, thus an electric current necessary for heating will decrease. On the other hand, when a flowing fluid quantity is large, an amount of heat being transferred from the flow rate detecting resistance 11 to the flowing fluid will also be large, thus an electric current necessary for heating will be increased. Thus, in a thermo-sensitive type flow rate sensor 1, an electric current for heating the resistance 11 is detected and used as a fluid rate signal, thereby detecting an actual flow rate of a fluid flowing through the main fluid passage 16 having a predetermined cross section area.
The thermo-sensitive type flow rate sensor 1, which is constructed in the above mentioned manner, is often used as an intake air flow rate sensor for an automobile engine, as shown in FIG. 38. Referring to FIG. 38, the flow rate sensor 1 is positioned within an intake air pipe 4 which is located downstream of an air cleaner element 2 enclosed in an air cleaner case 3. The air cleaner element 2 is a filter means made of a non-woven fabric or a filter paper, which is used to capture the dust entrained in the intake air so as to prevent it from entering the engine. However, after an automobile has been running for a certain long time, the air cleaner element 2 will get blocked due to the dust. Thus, an air flow having passed through the air cleaner element 2, when compared with a fluid having passed through a non-dust-blocked air cleaner element 2, will be more easily subjected to a change in the flow speed distribution of a fluid on the downstream side of the air cleaner element 2 before the fluid arrives at the flow rate sensor 1.
In fact, the flow rate detecting element 12 of the flow rate sensor 1 can detect only a part of the fluid flowing through the entire cross section of the main fluid passage 16. Accordingly, although the total quantity of a fluid flowing through the main fluid passage 16 does not change, a change in the flow speed distribution of a fluid on the upstream side of the flow rate sensor 1, will bring about an error to a flow rate detecting result.
In order to solve the above problem, it has been suggested that a fluid rectifying grating means 17 be provided in the main fluid passage 16 upstream of the flow rate sensor 1, as shown in FIGS. 36 and 37. Another conventional flow rate sensor has been disclosed in Japanese Unexamined Patent Publication No. 7-71985. In order to obtain a sufficient fluid rectifying effect, this conventional flow rate sensor employs a honeycomb structure, a net-like grating structure or a combination the honeycomb structure and the net-like grating structure.
Further, Japanese Unexamined Patent Publication Nos. 5-340778, 2-28520, 6-288805, have disclosed that a main fluid passage may be converged to have a Venturi shape as shown in FIG. 39, thereby obtaining a similar fluid rectifying effect.
Thus, a conventional flow rate sensor usually involves a fluid rectifying grating means 17 to rectify the fluid whose flow rate is to be measured. On the other hand, to obtain a sufficient rectifying effect, such kind of fluid rectifying means should be made so that the holes formed therethrough are quite small and that each unit area has a lot of such holes. However, since the fluid rectifying means has a honeycomb structure and since such rectifying means is required to have a sufficient rigidity, it is difficult to manufacture the fluid rectifying means with a lot of holes. As a result, a finally obtained fluid rectifying means has only a small aperture ratio (a small aperture area).
Further, since fluids flowing across many holes of a fluid rectifying means are unstable, a lot of small eddies will get together to form an irregularly large flowing of the fluid. As a result, there will occur a not uniform phenomenon in both the boundary layer thickness and the frictional stress around the detecting section of the flow rate sensor, hence causing fluctuations and errors in a flow rate detecting signal and thus making it impossible to perform a correct flow rate detection.
Moreover, it is understood that the ventilation resistance on the flow rate sensor 1 is large, hence an amount of intake air to be supplied to an automobile engine will be small, resulting in a problem that the automobile engine can only produce a small output power. In addition, since there are other fluid rectifying means in addition to the main fluid passage 16, the manufacturing cost is high.
Further, when a flow rate sensor employs a flow rate detecting element which is compact in size and capable of a quick response, and if a fluid rectifying means is positioned upstream of the flow rate detecting element, the flow rate sensor is likely to receive an undesired influence such as a turbulence caused by the fluid rectifying means. As a result, noise components possibly contained in a flow rate detecting signal will increase, making it difficult to perform the flow rate detection with a high precision.
On the other hand, in a conventional flow rate sensor which has been formed by using one section of the main fluid passage 16A converged into a Venturi shape, it is possible to obtain a sufficient fluid rectifying effect by setting a large converging ratio (a cross section area perpendicular to the main fluid axis at the inlet of the main fluid passage/a cross section area perpendicular to the main fluid axis at the narrowest converged portion).
However, when such a converging ratio is large, a fluid flowable cross section area will be small. Consequently, a ventilation resistance will be increased, resulting in a problem that the amount of an intake air being supplied to an internal combustion engine will be undesirably limited. Further, if such a converging ratio is large, the curvature of a curved surface forming a converged portion of the fluid main passage 16A will change rapidly, causing the direction of a fluid flowing therethrough to be suddenly changed. As a result, a fluid cracking phenomenon will occur in the flowing fluid, rendering a flow rate detecting signal to be unstable, hence making it impossible to perform a correct flow rate detection.
Moreover, since a portion of the fluid is stopped by a coupling portion of the fluid rectifying grating means 17 and the main fluid passage 16A, some dead fluid portions will occur in the fluid passage.